The effect of iron on mixed infection of Bacteroides fragilis and Escherichia coli in mice.

Survival of an infected animal is improved in a mixed infection when Bacteroides fragilis is the bacterial species acting as a microbial antagonist against Escherichia coli. The protective effect afforded an animal by B. fragilis against E. coli is altered when ferric ammonium citrate is added to the injected mixed culture. In mice an E. coli concentration of 10(9) produced zero survival; the concentration 10(8) produced 40% survival; the concentration 10(7) and less produced 100% survival. Bacteroides fragilis concentrations of 10(8) and less did not kill the 15- to 20-g mouse within 5 days. Mice were given ip injections of the constant amount of 10(8) E. coli and serially diluted amounts of B. fragilis beginning with 10(8) organisms/ml. There was 40% survival in the 10(8) E. coli controls and 96% survival in the animals receiving 10(8) E. coli plus B. fragilis. The addition of ferric ammonium citrate to 10(8) E. coli plus B. fragilis reduced animal survival from 96 to 63%.